Amplifier circuit



May 7, 1963 R. J. SMOLLETT, JR 3,089,100

AMPLIFIER CIRCUIT Filed June 5, 1959 INVENTOR. ROY J. SMOLLETT,-JR

AT ORNEY Staes Unite This invention relates to amplifier circuits andmore particularly to improved amplifier circuits wherein theconfiguration is such that variations in electron tube heater filamentvoltages are effectively balanced out to prevent their causing drift inthe amplifier.

In electric signal amplifier circuits, particularly in high gainamplifier circuits using electron tubes of the type having heaterfilaments, variations in voltage across the heater filaments, such ascaused by changes in power supply voltages, or changes in resistivevalues of the filaments themselves, result in undesirable drift effectsin circuit operation. This may be seen, for example, by visualizing asmall increase in average voltage across the filament of the first stageof a multistage high gain amplifier. The increased voltage results in ahotter filament with increased electron emission which, in turn, tendsto cause a decrease in voltage at the plate. The resulting decrease inplate voltage of the first stage may acquire a seriously large magnitudeat the amplifier output due to amplification in successive amplifierstages.

Pursuant to the present invention, this problem has been overcome by anovel amplifier circuit arrangement which also achieves other desirablefeatures and advantages. Among these other features and advantages arethe achievement of a relatively simple, inexpensive and reliable circuitarrangement which is self-contained and in itself automaticallycompensates for the filament voltage variations to thereby prevent tothe above-mentioned undesired drift.

Accordingly, a primary object of the present invention is the provisionof an amplifier substantially free of drift caused by variations infilament voltage.

Another object is the provision of an amplifier having a self-containedcircuit arrangement for automatically compensating for filament voltagevariations.

And a further object is the provision of a multistage high gainamplifier having a reliable yet relatively simple and inexpensivecircuit configuration which automatically compensates for variations inthe filament voltage.

These and other objects, features and advantages of the invention areachieved generally in an amplifier circuit of the type having a heaterfilament amplifier tube, with the filament coupled across the positiveand negative terminals of a filament power source, by providing acircuit coupled to the control grid and in responsive relation to thevoltage changes of the filament for effecting a grid voltage changewhich tends to balance out the effect of the filament voltage change.

By providing a resistive voltage divider circuit across the filamentvoltage source and coupled at a properly proportioned point to the grid,a simple inexpensive arrangement for automatic compensation of filamentchange is thereby achieved. By making the resistive circuit in the formof a voltage divider chain with one of the resistors in the chain beinga variable resistor, simplicity in adjusting balance setting is there-byachieved.

These and other features, objects and advantages of the invention willbecome more apparent from the following description taken in connectionwith the accompanying drawing of a preferred embodiment of the inventionand wherein:

The single FIGURE is a schematic drawing of a high 3,039,100 PatentedMay 7, 1963 gain amplifier made in accordance with the presentinvention.

Referring to the multistage high gain amplifier illustrated in thefigure, in more detail, there are three amplifier stages, 10, 12 and 14having anodes 16, 18 and 24) respectively coupled through resistors 22,24 and 26 to the positive terminal of a power source such as a battery28 having a negative terminal coupled to ground. The amplifier stages16, 12 and 14 also have screen grids 30, 32 and 34 respectively coupledto the positive terminal of grid biasing power source such as a battery36. The amplifier stages 10, 12 and 14 have control grids 38, 4t) and 42and heater filaments 44, 46' and 48 respectively. The respective stagesare coupled in cascade with the plate 16 of the first stage 10 beingconnected through a resistor 50 to the control grid 40 of the secondamplifier stage 12 which has its plate 18 coupled through a resistor 52to the control grid 42 of the third or output amplifier stage 14. Theplate 20 of the third amplifier stage 14 is coupled to an outputterminal 54. Control grids 40 and 42 are also coupled to the negativeterminal of a source of biasing potential such as a battery 56 having apositive terminal coupled to ground. A signal input terminal 58 iscoupled through a resistor 60 to the control grid 38 of the firstamplifier stage 10. The control grid 38 is also coupled through a chainof biasing resistors 62, 64 and variable resistor 66 to ground. Afeedback line 68 from the plate 20 of the output stage 14 is coupledthrough a resistor 76 and resistor 6-2 preferably of high resistivevalue to the control grid 38 and the first amplifier stage 10.

The heater filaments 44 and 46 of the first two amplifier stages 10 and12 are coupled in series across a power source such as a battery 72. Thepositive terminal of filament heater battery 72, is also coupled througha variable resistor 74 and fixed resistor 76' to the input terminal 58.The input terminal 58 is also coupled through a resistor 78 ground. Itwill be noted that the variable resistor 74, fixed resistors 76 and 78provide a voltage divider chain connected across the filament battery 72with the input terminal 58 being coupled to the voltage divider chainbetween the resistors 76 and 78. Also the variable resistor '74 andfixed resistors 76 and 60, 62, 64 and variable resistor 66 provideanother voltage divider chain with control grid 38 of the firstamplifier stage 10 being coupled to the voltage divider chain betweenresistors 6t} and 62. These two voltage divider chains are both ineffect coupled across the filament power source 72 and designed as willbe hereinafter explained to provide compensating voltage changes in thecontrol grid 38 for variations in voltage which may occur across theheater filament 44.

In the operation of the FIG. 1 high gain amplifier, input signals fromthe input terminal 58 will appear through resistor 60 at the controlgrid 38 of the first amplifier stage 10 thereby through resistor 50 atthe control grid 40 of the second amplifier stage 12 and similarlythrough the resistor 52 at control grid 42 of the third amplifier stage14, from the plate 20 of which they will appear at the output terminal54 suitably amplified in intensity. However, a problem which exists inconventional amplifier circuits of this nature is that in the event thevoltage across the heater filament 44 varies for any reason, such asvariations in voltages from the power source 72 or changes in resistivevalues of the heater filament 44 or heater filament 46, a correspondingfluctuation or drift occurs at the plate 16 of the amplifier stage 10.Such drift becomes increasingly serious as it becomes amplified in thesuccessive amplifier stages 12 and 14. For example, if the voltageacross the heater filament 44 increases, more current will flow throughthe filament 44 thereby increasing its temperature. Such 3 increasedtemperature of the filament 44 increases the electron emissivity of thefilament thereby increasing the electron flow to the plate 16 which, inturn, tends to cause a drop in voltage at the plate 16. However, in thepresent instance applicants above described voltage divider chainconstruction across the filament power source '72 efiectivelycompensates for such increased electron flow tendency from the filament44 by correspondingly regulating the voltage between the control grid 38and filament 44. This may be seen by assigning exemplary values to therespective resistors as follows: variable resistor 741,000 ohms,resistor 76-5,300 ohms, resistor 78-3,300 ohms, resistor 6047,000 ohms,resistor 62l megohm, resistor G L-3,300 ohms and resistor 66-100,000ohms. Now assuming a voltage of 2.8 volts across the filament battery72, the variable resistor 74 may be adjusted to provide a voltage at theinput terminal 58 of approximately 1.02 volts and a voltage at the grid38 of .96 volt. If for some reason the voltage across the filamentbattery increased, /2 of that increase would appear across the filament44 and /2 across the filament 46. On the other hand because of thevalues of voltage divider resistors 74, 76 and 78 a much smallerincrease than the abovementioned /2 would occur at the input terminal58. Because of the voltage divider chain resistors 60, 62, 64 and 66 aneven smaller change than that at the input terminal 58 would appear atthe control grid 38. It has been found that for the presentconfiguration the proportion values of resistors is such that the changeat the control grid 38 caused by the fluctuation of the voltage acrossthe heater filament 44 is just suflicient to provide a relative voltagebetween control grid 38 and filament 44 to make the control grid 38 looksulficiently more negative to the average filament 44 voltage to justcompensate for and prevent the increased electron flow which wouldotherwise occur from increased temperature of the filament 44. Whilethese resistor values have been found to satisfactorily compensate forsuch fluctuations in the present embodiment, they are cited here by wayof illustration only and other proportions and values may be assignedfor other amplifier conditions by using the principles and concepts heredisclosed. Adjustment of variable resistor 66 provides a suitablearrangement for varying the gain of the amplifier stage 10. Adjustmentof variable resistor 74 provides a rapid and suitable arrangement forthe finding of the initial compensating setting for the voltage dividerchain across the filament power source 72.

This invention is not limited to the specific construction and operationherein described asequivalents will suggest themselves to those skilledin the art.

What is claimed is:

1. In an amplifier stage, the combination of at least two activeelectrodes, including a filament-type cathode and a control grid, aresistance chain with the control grid coupled through the resistancechain to ground, a source of uni-directional electric energy in thestage, the source having a pair of terminals and being arranged for.heating the cathode and having one of the terminals coupled to ground,and resistance means coupling said other terminal to a point on thevoltage divider chain such that variations in voltage across the sourceof unidirectional electric energy will cause proportional correctivevoltage changes at the grid to maintain a substantially constant currentflow in the stage.

2. In an amplifier stage, the combination of at least two activeelectrodes, including a filament-type cathode and a control grid, aresistance chain with the control grid coupled through the resistancechain to ground, a single battery in the stage coupled across thefilament and having one side coupled to ground, and variable rea sistormeans coupling the other side of the battery to a point on the voltagedivider chain such that variations in voltage across the battery willcause proportional corrective changes at the grid to maintain asubstantially constant current flow in the stage.

3. In an amplifier circuit, the combination of a first and secondamplifier stage, including a filament-type cathode having a heater and afilament control grid in each stage with the control grid of the secondstage coupled to the output of the first stage, a resistance chain withthe control grid of the first stage coupled through the resistance chainto ground, a source of uni-directional electric energy and having a pairof terminals with one of the terminals coupled to ground, the filamentscoupled in series across the uni-directional source of electric energy,a voltage divider resistor chan coupled across the uni-directionalsource of electric energy in parallel with the filaments, and resistormeans coupling the other terminal to a point on the divider chain suchthat variations in voltage across the source of uni-directional electricenergy will cause corrective voltage changes at the first grid tomaintain a substantially constant current flow in the stages.

4. In an amplifier circuit, the combination of a first and secondamplifier stage, including a filament type cathode and a control grid ineach stage with the con- 'trol grid of the second stage coupled to theoutput of the first stage, a resistor chain with the control grid of thefirst stage coupled through the resistor chain to ground, a batteryhaving positive and negative terminals, the filaments coupled in seriesacross the battery and the negative terminal coupled to ground, andresistor means coupled to the positive terminal of the battery and to apoint on the resistor chain such that variations in voltage across thebattery will cause corrective voltage changes at the first grid tomaintain a substantially constant current flow in the stages.

References Cited in the file of this patent UNITED STATES PATENTS1,794,708 Zoziers Mar. 3, 1931 2,601,583 Ballou June 24, 1952 2,693,572Chase Nov. 2, 1954 2,759,050 Lord Aug. 14, 1956 2,764,641 Muschamp Sept.25, 1956 FOREIGN PATENTS 518,640 France Jan. 7, 1921

4. IN AN AMPLIFIER CIRCUIT, THE COMBINATION OF A FIRST AND SECONDAMPLIFIER STAGE, INCLUDING A FILAMENT TYPE CATHODE AND A CONTROL GRID INEACH STAGE WITH THE CONTROL GRID OF THE SECOND STAGE COUPLED TO THEOUTPUT OF THE FIRST STAGE, A RESISTOR CHAIN WITH THE CONTROL GRID OF THEFIRST STAGE COUPLED THROUGH THE RESISTOR CHAIN TO GROUND, A BATTERYHAVING POSITIVE AND NEGATIVE TERMINALS, THE FILAMENTS COUPLED IN SERIESACROSS THE BATTERY AND THE NEGATIVE TERMINAL COUPLED TO GROUND, ANDRESISTOR MEANS COUPLED TO THE POSITIVE TERMINAL OF THE BATTERY AND TO APOINT ON THE RESISTOR CHAIN SUCH THAT VARIATIONS IN VOLTAGE ACROSS THEBATTERY WILL CAUSE CORRECTIVE VOLTAGAE